SAN FRANCISCO — For years, Uber and Lyft put off going public. Now, they are speeding up.

Faced with a volatile stock market and the prospect of an economic downturn next year, the ride-hailing services have moved more urgently toward an initial public offering, said four people with knowledge of the companies’ plans, who were not authorized to speak publicly.

Lyft originally aimed to list its shares toward the middle of 2019, but it began moving more quickly after the recent stock market sell-off and because of a desire to go public before Uber, said two of the people. On Thursday, the company, which was most recently valued at $15 billion, announced it had filed confidentially for an I.P.O.

Uber has also hastened its I.P.O. clock. The company had once said it was looking to the fall of 2019 to go public, but has pushed that timing up because of concerns that a recession might be coming, said two people familiar with the plans. Uber could now go public as soon as next April, they said. Investment banks have told the company it could be worth as much as $120 billion in an I.P.O.

The moves by Lyft and Uber indicate how tricky it can be to decide when to go public at a time when stock markets have been turbulent and the broader economic picture is muddied. The calculus for when a company publicly lists its shares is often a moving target, but Uber’s and Lyft’s actions will carry particular weight with a swath of other highly valued Silicon Valley start-ups that are also preparing to approach the public markets.

Airbnb, the online room rental company, plans to be ready to go public by mid-2019 though it has not set a formal timeline, said a person with knowledge of the matter. Slack, the online collaboration company, has said it is readying for a public offering but has no specific timeline.

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Airbnb plans to be ready to go public by mid-2019, though it has not set a formal timeline.CreditToshifumi Kitamura/Agence France-Presse — Getty Images

“Companies that were talking about 2020 have been told that the window may not be open as long as previously thought,” said Barrett Daniels, a partner at Deloitte who advises on I.P.O.s. He said that he was telling companies that “if an I.P.O. is in your plan, I would probably be getting ready now.”

Any stock market debuts of these companies will be the final chapter of the era of “unicorns,” the privately held start-ups valued at more than $1 billion. Many of these companies, which were born after the 2008 recession, rode a wave of smartphone adoption, turning businesses like taxis or grocery delivery into on-demand services. They also benefited from abundant capital from private investors, which was driven by low interest rates.

For years, many unicorns were in no rush to go public because they could grow easily with money from private investors and away from the scrutiny of Wall Street. In 2016, Travis Kalanick, Uber’s co-founder and then chief executive, spoke for many tech start-ups when he said at a conference that his company would go public “as late as humanly possible.” Employees would be distracted by stock price movements, he said.

Those attitudes have shifted as investors and tech employees have increased pressure on the companies to go public so that they can cash in their shares.

“The forcing factor is, how do you deal with issues of employee retention?” said Rick Heitzmann, a managing director at FirstMark Capital, which is an investor in unicorns such as Pinterest and Airbnb.

But the seesawing stock market, a trade war with China and other countries and uncertainty over the direction of the economy are all now weighing on I.P.O. decision-making. Few executives want to take their companies public when investors’ appetite for shares may be ebbing.

“Companies that were waiting for everything to be perfect before going public might have been better off going when things were good enough,” Mr. Heitzmann said.

Sandy Miller, a venture capitalist at IVP, said several companies were meeting with potential investors far ahead of filing for an I.P.O., in what are known as pre-roadshow events. “That’s the only way to really know what kind of receptivity you’re going to have” from the public markets, he said.

Mr. Miller said that he expected a robust year for I.P.O.s next year, but that companies may not want to wait until too late in the year to file. “There are certainly some storm clouds on the horizon,” he said.

Some unicorns are sidestepping the unpredictability altogether. WeWork, an office rental company valued at $45 billion, has been widely named as an I.P.O. candidate. But in November, the company agreed to sell an additional $3 billionof shares to its main investor, SoftBank’s Vision fund. That deal has allowed WeWork to push plans for a public listing further into the future, said a person familiar with the company.

For Uber and Lyft, the biggest question they face from public market investors is whether their businesses can be profitable. Expanding a ride-hailing service requires outlays to recruit drivers in multiple cities, which can quickly get expensive. Uber said last month that it lost $1.07 billion in the third quarter, as it spent to invest in new areas such as bicycles, scooters and freight shipments.

Inside Uber, Dara Khosrowshahi, the chief executive, has raced to prepare the company to go public. Over the past year, the company has overhauled many of its internal processes, from items as small as creating more formal systems for expense reports to global safeguards that ensure legal compliance in every area where Uber operates.

Going public sooner could give Uber a number of advantages. It would mean raising fresh outside capital, providing the company ammunition to pursue acquisitions and other opportunities. And it would enable Mr. Khosrowshahi to potentially reshape Uber’s board because current members can be asked to leave when there is a liquidity event like an I.P.O., according to the company charter. Uber’s board has grappled with a history of infighting.

Lyft is likely to still go public ahead of Uber because it has already filed for an offering. In a statement on Thursday about its confidential I.P.O. filing, Lyft said it had not yet determined how many shares would be sold or their price range.

Any offering would be “subject to market and other conditions,” Lyft said.